M. A. Averkiou
L. A. Crum
Appl. Phys. Lab., Univ. of Washington, Seattle, WA 98105
M. F. Hamilton
Univ. of Texas at Austin, Austin, TX 78712-1063
A computer code that solves the KZK equation in the time domain [Lee and Hamilton, Ultrasonics Intl. 91 Conference Proceedings (Butterworth-Heinemann, Oxford, 1991), pp. 177--180] is modified to include the effects of focusing on pulsed sound beams of finite amplitude. Single cycle pulses, similar to those used in lithotripsy and high intensity focused ultrasound (HIFU) surgery, are considered. The numerical results demonstrate the combined effects of nonlinearity, absorption, diffraction, and focusing on the waveform distortion and shock formation in short acoustic pulses. Self-defocusing associated with shock formation is also taken into account. Propagation curves are shown for the positive peak pressure amplitude (P[sub +]) and the negative peak pressure amplitude (P[sub -]) along the axis of the source. The calculated pressure waveforms are used as input to computer codes that model bubble dynamics. Predicted radius-time curves are presented for the response of a bubble due to a focused acoustic pulse of finite amplitude. Gas diffusion from the liquid into the vapor bubble, and the roles of P[sub +] and P[sub -] in the diffusion process, are also considered. [Work supported by the National Institute of Health.]